Field of the Invention
The invention relates generally to linear permanent magnet motors, and more particularly the means for monitoring the force produced by a linear permanent magnet motor, determining the existence of a pump-off condition, and controlling the motor to prevent fluid pound resulting from pump-off.
Related Art
In the production of oil from wells, it is often necessary to use an artificial lift system to maintain the flow of oil. Artificial lift systems may utilize various types of pumps to lift oil out of the well. For instance, some conventional installations use rod lift systems in which a surface motor moves a sucker rod up and down in a reciprocating motion to drive a linear pump in the well. Other installations use electric submersible pumps (ESP's) in which a motor and a pump are positioned downhole in the well to pump oil out of the well.
In an artificial lift system that uses a linear permanent magnet motor to lift oil from the well, it may be useful to be able to determine the amount of force that would be produced by the system. This information may be used, for example, to adjust the speed at which the system's motor is operated. The amount of force being produced by the motor may also be used to determine whether the pump has encountered a gas pocket, or is approaching a pump-off state. Pump-off occurs due to a change in the fluid volume of the well under operation. This change in the fluid volume of the well is a natural phenomenon, and may occur due to several reasons, such as a change in the reservoir level, artificial stimulation of the well for a long time, etc. During pump-off, most of the fluid column could be filled with gas rather than liquid/oil. Under these circumstances, the mover may encounter sections of empty space/gas pockets through which it will act as a free-falling body before it encounters any fluid, which will occur with a greater-than-expected pound or force. The sudden, dramatic change in the forces/load within a stroke (referred to as fluid pound) may cause tremendous stresses on the ESP string, and may also damage the motor or shorten its useful life.
In a traditional pump system that uses a sucker rod, it is necessary to incorporate a strain gauge or some other type of load cell into the system to measure the pumping force. This load measuring component of the system requires a force measurement device, along with a physical assembly on the sucker rod pump. This makes the entire system more bulky and expensive.
It would therefore be desirable to provide systems and methods for determining the force generated by an ESP that is operated by a linear permanent magnet motor, detecting pump-off conditions based on the force, and controlling the motor to prevent fluid pound that may result from pump-off.